This invention relates to a TFT array substrate for a liquid crystal display device in an active matrix mode, using a thin film transistor.
In recent years, there have been actively developed liquid crystal display devices in an active matrix mode in which a polysilicon thin film transistor (referred to as a poly-Si TFT hereinafter) of a low temperature processed type is used as a control element instead of an amorphous silicon thin film transistor. The reasons thereof are as follows: the exactitude of liquid crystal display devices can be made higher and their numerical aperture can be made higher because the poly-Si TFT has a larger electric field-effect mobility than the amorphous silicon TFT; and it may be possible to provide liquid crystal display devices having a large area and a high exactitude at low cost since the low temperature processed type makes it possible to use an inexpensive glass substrate.
Referring to FIG. 7, a method for producing such a low temperature processed poly-Si TFT will be described. In FIG. 7, 701, 702, 703, 704 and 705 represent a glass substrate, a buffer layer, an amorphous silicon layer, a polysilicon layer and a gate insulator layer, respectively. 706, 707, 708, 709, 710 and 711 represent a gate electrode, a source area, a drain area, contact holes, a source electrode and a drain electrode, respectively.
In the process for production thereof, the buffer layer 702 made of a Si3N4, for example, 600 xc3x85 in thickness is first formed on the glass substrate 701. Amorphous silicon is deposited on the entire surface of this buffer layer 702 (FIG. 7(a)). Next, the entire surface of this amorphous silicon layer 703 is irradiated with an excimer laser to heat and melt the silicon and re-crystallize it. In this way, the polysilicon layer 704 is made. A Si3N4 layer, for example, 200 xc3x85 in thickness and a SiO2 layer 1500 xc3x85 in thickness are vapor-deposited on the polysilicon layer 704 to form the gate insulator layer 705. The gate electrode 706 made of Mo, for example, 6000 xc3x85 in thickness is formed on the gate insulator layer 705. Phosphorus ion is implanted into the polysilicon layer 704 through the gate electrode 706 as a mask (FIG. 7(b)). Irradiation with an excimer laser is again conducted to activate the phosphorus ion implanted into the polysilicon layer 704. In this way, the source area 707 and the drain area 708 are made (FIG. 7(c)). At last, the gate insulator layer 705 is etched to make the contact holes 709/709 reaching the source area 707 and the drain area 708. In this way, the source electrode 710 and the drain electrode 711 each having a thickness of 3000 xc3x85, in which Al is embedded in the contact holes 709/709, are formed. By the above-mentioned process, a low temperature processed poly-Si TFT is completed.
In this method, a rise in the temperature of the substrate is a little (about 600xc2x0 C. or lower) since the excimer laser is used for polycrystallization. Therefore, an inexpensive substrate can be used, and a polysilicon thin film having a larger area can be formed, as compared with high temperature processing method (about 1000xc2x0 C.). Thus, the screen of liquid crystal display devices can be made large.
However, if the above-mentioned low temperature processing method is used to produce a liquid crystal display device having a large screen, display unevenness becomes large. At present, sufficient display performance cannot be realized.
A main object of the present invention is to overcome the above-mentioned problems in a low temperature processed poly-Si TFT in the prior art. More specifically, it is an object to provide a poly-Si TFT array substrate having a high electric field-effect mobility and a little in-plane dispersion without using an expensive quartz substrate. It is another object to use such a poly-Si TFT array substrate so as to provide a more inexpensive liquid crystal device having a large screen, a high exactitude and a high performance.
Before disclosure of the constitution of the present invention for attaining the above-mentioned objects, the cause of generation of display unevenness in low temperature processed poly-Si TFTs in the prior art will be discussed.
FIG. 4 is a schematic plan view of a TFT array substrate. In FIG. 4, 412 and 413 represent a glass substrate, and a pixel area formed on the glass substrate 412, respectively. In this pixel area 413, non-illustrated pixels are arranged in a matrix form and non-illustrated pixel switching TFTs are arranged so as to correspond to the respective pixels. 414 and 415 represent so-called peripheral driving circuits for driving the pixel-switching TFTs. For example, 414 represents a gate driving circuit unit and 415 represents a source driving circuit unit having therein the TFTs.
As shown in FIG. 7, in conventional methods, amorphous silicon is deposited on the entire surface of the glass substrate 412. Thereafter, the substantially entire surface of the amorphous silicon layer is irradiated with an excimer laser to melt and polycrystallize the silicon. According to this method, however, the following problems arise.
That is, since the width of an excimer laser is limited, a large area is not irradiated with the laser at once. Thus, a method in which a linear excimer laser (line beam) is successively scanned on a substrate is adopted. However, according to this method, long and narrow crystal grains are produced along the line direction of the line beam. Furthermore, the shapes and the sizes of the crystal grains easily become nonuniform since this method is a method in which the crystallization is successively attained. The amorphous silicon layer has no crystal nuclei for inducing crystal growth at the initial stage of crystallization. Thus, at some stage that the excimer laser is applied and the crystallization starts, crystal nuclei are generated uncertainly and disorderedly so that crystal grows rapidly. Therefore, the crystal growth becomes unstable and disordered so that the shapes and the sizes of the crystal grains become nonuniform. Moreover, grain boundaries where very small crystal grains collide with each other swell and the structure of the grain boundary portions are distorted since the crystal grows rapidly.
In reality, the inventors scanned an excimer laser (line beam) on a 320 mmxc3x97400 mm amorphous silicon layer to carry out polycrystallization, and then examined the electric field-effect mobility of respective sites of this polysilicon layer. As a result, it was verified that the electric field-effect mobility varied within the range of 50 to 300 cm2/V-s, dependently on the sites. The following tendency was also verified: polysilicon at the peripheral area had a higher electric field-effect mobility than polysilicon at the center.
In other words, according to the method for producing a low temperature processed poly-Si TFT which has been hitherto known, the electric field-effect mobility of its polysilicon layer becomes nonuniform. This tendency becomes remarkable, particularly in TFTs formed in an array form in a pixel area. This would cause display unevenness (for example, linear unevenness). On the other hand, according to a high temperature processing polycrystallization method (about 1000xc2x0 C. or higher), which is performed using an expensive quartz substrate, the above-mentioned problems of the low temperature process method are easily solved. However, according to the high temperature processing method, it is difficult that a large screen is produced. Moreover, a problem that costs increase arises.
The present invention for solving problems as describe above has the following constitutions. The present invention is classified to from a first invention group to a seventh invention group, and these groups will be successively described.
(1) First Invention Group
A first embodiment of the present invention concerned in the first invention group is a method for producing a TFT array substrate for a liquid crystal display device, comprising a process of forming, on a substrate, a poly-Si TFT in which a polysilicon semiconductor layer is used in a channel area, characterized by comprising a polysilicon layer forming step of depositing silicon particles excited by adding energy beforehand onto the substrate so that the polysilicon layer is formed at the stage when the silicon particles are deposited on the substrate.
According to this constitution, it is possible to form a polysilicon layer having a uniform electric field-effect mobility in its plane even if the layer has a large area. The reason thereof is as follows.
The method for forming polysilicon layer in the prior art is a method of depositing amorphous silicon on a substrate and then heating and melting the amorphous silicon to recrystallize the silicon. In this way, a polysilicon layer is formed. However, according to this method, crystal nuclei are generated uncertainly and disorderedly at an initial stage after the heating and melting, so that the shapes and the sizes of the crystal grains become nonuniform. Thus, such a problem that electric field-effect mobility varies widely arises.
On the contrary, the above-mentioned constitution is a production method wherein by using silicon particles to which energy is added, the silicon particles turn to a polysilicon layer at the stage when the silicon particles are deposited on a substrate. Since this method does not comprise the step of heating and melting amorphous silicon to recrystallize the silicon, such a problem caused in the conventional low temperature processing method does not arise. This fact will be further described. According to the above-mentioned constitution, the silicon particles to which the energy is added have an energy over a regular level for some time after the particles reach the substrate. Therefore, the particles migrate on the substrate so that they move to a stable point where their energy state is more stabilized. By such movement, polycrystallization of the deposited layer advances. However, during the advance of the polycrystallization, new silicon particles are successively deposited so that the particles migrate on the substrate. Accordingly, even if defects or the like are generated in crystal structure, the crystal defects or the like are cured with the newly added silicon particles. Thus, according to the above-mentioned constitution, it is possible to form crystal grains having a small amount of the crystal defects and form a polysilicon layer having a uniform density and a good quality.
According to the low temperature processing method in the prior art, temperature distribution becomes nonuniform if the area of a substrate becomes large. For this reason, it is difficult to form a polysilicon layer having a good quality. The production method of the present invention is, however, a method of attaining polycrystallization at the same time when silicon particles to which energy is added are successively deposited on a substrate. Accordingly, a polysilicon layer having a uniform quality can be formed without being affected by the size of the area of the substrate. Moreover, productivity is good since heating and melting are unnecessary.
A second embodiment concerned in the first invention group is characterized in that after the polysilicon layer forming step in the first embodiment, a heating-treatment step for heating and melting the polysilicon layer formed in the polysilicon layer forming step to recrystallize the polysilicon is further added.
According to this constitution that the polysilicon layer formed in the polysilicon layer forming step is subjected to the heating-treatment, the polysilicon layer can be made to have a very high electric field-effect mobility. The reason thereof is as follows. When the polysilicon layer is heated, small crystal grains are melted to become a melted product to be recrystallized. However, large crystal grains are not melted completely to remain as fine grains. They turn to crystal nuclei for recrystallization. Thus, the recrystallization advances smoothly. As a result thereof, it is possible to form a recrystallized polysilicon layer which is a product in which crystal grains made of uniform and large grains gather. Since such a polysilicon layer has a high electric field-effect mobility, a poly-Si TFT having high rapidity can be produced.
A third embodiment concerned in the first invention group is characterized in that the heating-treatment in the heating-treatment step is conducted in an atmosphere comprising hydrogen.
When the heating-treatment is conducted in the hydrogen atmosphere, dangling bonds of silicon are terminated. Therefore, the electric field-effect mobility of the polysilicon layer can be further improved.
A fourth embodiment concerned in the first invention group is characterized by comprising, after the polysilicon layer forming step, the steps of producing a pixel-switching poly-Si TFT and producing a driving poly-Si TFT for driving the pixel-switching poly-Si TFT.
Since the polysilicon layer formed in the polysilicon layer forming step has a high electric field-effect mobility, the TFT having this polysilicon layer as a channel area has a superior rapidity. The TFT can be appropriately used as not only a pixel-switching element but also an element for a driving circuit of the pixel-switch. Thus, according to the above-mentioned production method for producing both of a pixel-switching poly-Si TFT and producing a poly-Si TFT for driving this on a single substrate, a TFT array substrate for a liquid crystal display device having a superior rapidity and a superior integration degree can be efficiently produced.
A fifth embodiment concerned in the first invention group is characterized in that the fourth embodiment comprises a specific area heat-treating step of selectively heat-treating only a specific area where the poly-Si TFT for driving should be formed before the step of forming the poly-Si TFT for driving, to increase crystallinity of the polysilicon layer in this area.
According to this constitution, only the specific area where the poly-Si TFT for driving should be formed is beforehand heat-treated to conduct recrystallization. In this method, a TFT array substrate having superior rapidity can be efficiently produced. This is because the driving TFT needs higher rapidity than any pixel-switching TFT. On the other hand, the specific area where the TFT for the peripheral driving circuit should be formed has a smaller area than the whole area of the substrate. Therefore, if the above-mentioned constitution that only the circuit portion which needs higher rapidity is heated-treated, recrystallization can be attained at a smaller energy. Nonuniformity is not easily caused in temperature distribution by the small of the heated area. Therefore, uniform crystallization can be attained.
A sixth embodiment concerned in the first invention group is characterized in that in the fifth embodiment, an excimer laser or an infrared ray lamp is used as a heating means in the specific area heat-treating step. The excimer laser or the infrared ray lamp is preferable since it makes partial heating possible and has good heating efficency.
Referring to the above-mentioned FIG. 4, the fifth embodiment and the sixth embodiment will be further described.
FIGS. 4(a) and 4(b) are plan views showing an outline of an ordinary TFT array substrate for a liquid crystal display device. The difference between FIG. 4(a) and FIG. 4(b) is that the areas of the display portions of the liquid crystal display devices are different. That is the display area in FIG. 4(b) is larger than that in FIG. 4(a). In the conventional method of forming an amorphous silicon layer on a substrate and subsequently subjecting the resultant to excimer laser annealing to attain crystallization, an excimer laser beam must be applied onto the substantial entire surface of the substrate. At present, however, there are not devices in which an excimer laser beam can be applied at a time onto a large area. Thus, a method of scanning a linear excimer laser beam successively is adopted. According to this method, however, productivity is poor and it is difficult to obtain a uniform polysilicon layer. According to the method of additionally using an infrared ray lamp for irradiating the entire surface with infrared rays, the temperature of the substrate rises. Therefore, inexpensive glass substrates cannot be used.
On the other hand, according to the fifth embodiment and the sixth embodiment, such problems as in the prior art cannot arise. First, this is because the layer formed on the substrate is a polysilicon layer from the beginning. Second, this is because only the specific area limited onto the substrate is heat-treated (recrystallized).
As is clear from the comparison between FIGS. 4(a) and 4(b), the width of the driving circuit portions is not very much affected by the size of the display portions. Thus, if only the driving circuit portion is recrystallized, recrystallization can be attained without use of especial excimer laser applying device. Since the other portion (the pixel portion) is made of a polysilicon layer, the it has sufficient electric field-effect mobility even if it is not recrystallized. Furthermore, recrystallization of a polysilicon layer makes it possible to gain a higher quality than crystallization of an amorphous layer. For these reasons, according to the fifth embodiment and the sixth embodiment, uniform transistor characteristics can be obtained on the entire surface of the array substrate.
If, in FIG. 4(a), scanning directions 433a and 433b at the time of applying line beams 432a and 432b of an excimer laser while scanning these beams are made parallel to scanning directions of source and gate signals, efficient recrystallization can be performed even when the laser has a short beam width.
A seventh embodiment concerned in the first invention group is characterized in that in the sixth embodiment, the heat-treatment in the specific area heat-treating step is conducted in an atmosphere containing hydrogen. In the case that the heat-treatment is conducted in the atmosphere containing hydrogen, dangling bonds of silicon can be terminated and the electric field-effect mobility of the polysilicon layer can be further improved. Thus, this case is preferable.
An eighth concerned in the first invention group is characterized in that in the fifth embodiment, the heat-treatment is conducted so that the electric field-effect mobility of the specific area will be 100 cm2/V-s or more. When the electric field-effect mobility is made 100 cm2/V-s or more, high-frequency driving can be attained.
A ninth embodiment concerned in the first invention group is characterized in that the second embodiment comprises, after the heat-treating step, the step of producing a pixel-switching poly-Si TFT for switching a pixel, and an IC chip integrating step of integrating a monocrystal silicon IC chip having therein a circuit for driving the pixel-switching poly-Si TFT produced in the above-mentioned production step into the substrate.
A poly-Si TFT makes far faster switching possible than an amorphous Si type TFT. Therefore, if a monocrystal silicon IC chip which can be driven by high frequency is combined, as a driving element, with this pixel-switching poly-Si TFT, it is possible to produce a TFT array substrate for a liquid crystal display device in which high-speed operating performance of the monocrystal silicon IC chip can be sufficiently utilized to exhibit superior response speed.
A tenth embodiment concerned in the first invention group is characterized in that the polysilicon layer forming step in the first embodiment is made to the step of applying thermal energy to an vapor source comprising solid silicon to vaporize silicon to prepare silicon particles; exciting and ionize the silicon particles in a plasma atmosphere, and then depositing the excited silicon particles onto the substrate.
According to this constitution, the vapor source comprising silicon which is the same material as constitutes the polysilicon layer is used so that the polysilicon is formed. Thus, no impurities are incorporated into the polysilicon layer. According to this method using the vapor source, the area where the silicon particles are generated can be made wide and the silicon particles can be deposited from various directions onto the surface of the substrate. Thus, the formed polysilicon layer can be made superior in uniformity. This advantage can be remarkably exhibited, particularly in the case that a polysilicon layer having a large area is formed.
Furthermore, according to this constitution, the silicon particles are excited and ionized in the plasma atmosphere and then the particles are deposited onto the surface of the substrate to form a deposition layer. The silicon particles deposited in this method keep energy after they reach the substrate so that they can migrate to stable points where the state of energy can be more stabilized on the substrate. Thus, if defects are generated in the crystal during the step of crystallization, newly deposited silicon particles migrate to cancel the defects. According to the production method of this constitution, a polysilicon layer of minute crystal having a few defects is formed by such movement of the silicon particles. Such a polysilicon layer is superior in transistor characteristics.
An eleventh embodiment concerned in the first invention group is characterized in that in the tenth embodiment, the substrate in the polysilicon layer forming step is arranged outside the plasma atmosphere. If the substrate is arranged inside the plasma atmosphere, plasma particles collide with each other so that the temperature of the substrate rises. However, according to the above-mentioned constitution in which the substrate is arranged outside the plasma atmosphere, such a problem does not arise. Thus, an inexpensive glass substrate low in heat-resistant temperature can be used.
A twelfth embodiment concerned in the first invention group is characterized in that in the eleventh embodiment, the substrate in the polysilicon layer forming step is arranged in a direction different from a direction along which the silicon particles are vaporized from the vapor source.
According to this constitution, the vaporized silicon particles move once so as to be apart from the substrate. Thereafter, only the excited and ionized particles having high energy are applied on the surface of the substrate.
A thirteenth embodiment concerned in the first invention group is characterized in that the polysilicon layer forming step in the first embodiment is a step of exciting and ionizing the silicon particles generated by decomposing a gaseous silicon compound with high frequency energy in the plasma atmosphere, and then depositing the excited silicon particles onto the substrate.
Even in the process of decomposing the gaseous silicon compound to produce the silicon particles, the effect and advantage described in the first embodiment can be obtained. However, in this method in which the silicon compound is decomposed, productivity is poorer and impurities are more easily incorporated into the formed polysilicon layer than in the method in which the silicon particles are generated from the solid siliconvaporation source.
A fourteenth embodiment concerned in the first invention group is characterized in that in the thirteenth embodiment, the substrate in the polysilicon layer forming step is arranged outside the plasma atmosphere. According to this constitution, the same effect and advantage as in the eleventh embodiment can be obtained.
A fifteenth embodiment concerned in the first invention group is characterized in that in the 11th, 12th, 13th or 14th embodiment, a means for applying an electric field is disposed between the plasma atmosphere and the substrate and the silicon particles excited and ionized in the plasma atmosphere are pulled out by the electric field and applied onto the substrate.
According to this constitution, only the ionized particles in the plasma atmosphere are pulled out from the silicon particles excited and ionized in the plasma atmosphere with the electric field applying means and then they are applied onto the substrate. Since the ionized particles have high energy levels, they migrate actively on the substrate to form a better-quality polysilicon layer. Thus, a poly-Si TFT having higher rapidity can be formed.
A sixteenth embodiment concerned in the first invention group is characterized in that in the tenth embodiment, the polysilicon layer forming step is a step of using a pressure gradient type plasma gun comprising a silicon particle generating means for applying arc discharge energy onto the vapor source comprising the solid silicon to vaporize silicon and produce the silicon particles and an exciting means for introducing the produced silicon particles to the plasma atmosphere to excite the particles and produce the ionized particles, so as to produce the excited and ionized silicon particles; and depositing the silicon particles onto the substrate.
If the above-mentioned pressure gradient type plasma gun is used, it is possible to form very efficiently a polysilicon layer having high quality, in particular to form efficiently a polysilicon layer having a large area.
(2) Second Invention Group
A seventeenth embodiment of the present invention, concerned in the second invention group, is a method for producing a TFT array substrate for a liquid crystal display device, having a process for producing a TFT on a substrate, characterized by comprising a gate insulator layer forming step of applying thermal energy to a solid vapor source comprising the same material as constitutes a gate insulator layer to vaporize the material to prepare particles; and exciting and ionizing the particles in a plasma atmosphere to be applied and deposited onto the substrate, so as to form the gate insulator layer on a silicon semiconductor layer of a channel area in the TFT.
The gate insulator layer forming step is an evaporation method having the same principle as the polysilicon layer forming method described in the first invention group. In this evaporation method, the very same material as constitutes the gate insulator layer is used as the vapor source and the particles vaporized from this vapor source are deposited to make the gate insulator layer. Therefore, impurities of the formed gate insulator layer can be made to be very small amount. According to the production method of the above-mentioned constitution, the gate insulator layer can be successively formed on the polysilicon layer without exposing to the atmosphere the silicon layer as an active layer of the TFT by adopting a load lock manner. The interface between the silicon layer and the gate insulator layer can be completely prevented from being polluted.
Furthermore, according to the production method of the above-mentioned constitution, a uniform and minute gate insulator layer can be formed in the same way as in the case of the polysilicon layer. As a result thereof, a TFT array substrate in which transistor characteristics hardly vary can be produced.
(3) Third Invention Group
An eighteenth embodiment concerned in the third invention group is a method for producing a TFT array substrate for a liquid crystal display device, having a process for producing a TFT on a substrate, characterized by comprising a gate insulator layer forming step of decomposing a gaseous compound containing the same atom as constitutes a gate insulator layer with high frequency energy to generate particles containing the same atom, and exciting and ionizing the particles containing the same atom in a plasma atmosphere and depositing the particles onto the substrate to form the gate insulator layer on a silicon semiconductor layer of a channel area in the TFT.
This constitution is a matter in which the same principle as in the thirteenth embodiment of the first invention group is used for the formation of the gate insulator layer. By this constitution, it is possible to form TFTs in which Vt characteristics (operating threshold voltage of transistors; threshold voltage) hardly vary.
(4) Fourth Invention Group
A nineteenth embodiment concerned in the fourth invention group is a method for producing a TFT array substrate for a liquid crystal display device, having a process for producing a poly-Si TFT on a substrate, characterized by comprising the step of applying thermal energy to an vapor source comprising solid silicon to vaporize silicon to prepare silicon particles, and exciting and ionizing the particles in a plasma atmosphere and depositing the particles onto the substrate to form a polysilicon layer on the substrate; and a gate insulator layer forming step of applying thermal energy to a solid vapor source comprising the same material as constitutes a gate insulator layer to vaporize the material to prepare particles, and exciting and ionizing the particles in a plasma atmosphere and depositing the particles onto the substrate to form the gate insulator layer.
According to this constitution, it is possible to produce TFTs having a high electric field-effect mobility and less varied Vt characteristics with high productivity.
A twentieth embodiment concerned in the fourth invention group is characterized in that in the nineteenth embodiment, as a device for performing the polysilicon layer forming step and the gate insulator layer forming step, a pressure gradient type plasma gun is used, the plasma gun comprising a particle generating means for applying arc discharge energy onto an vapor source comprising a solid material to vaporize the vapor source to generate particles and an exciting means for introducing the generated particles into the plasma atmosphere to be excited and ionized.
If the above-mentioned pressure gradient type plasma gun is used, vaporized particles can be efficiently generated. The area where the particles are vaporized can also be made wide. Therefore, it is possible to form uniform thin films whose film densities vary a little. The effect and advantage are remarkably exhibited particularly in the case that the area of the thin films becomes large.
(5) Fifth Invention Group
A twenty first embodiment concerned in the fifth invention is a TFT array substrate for a liquid crystal display device, comprising at least a transparent pixel electrode, a pixel-switching TFT for switching the transparent pixel electrode and a driving element for driving the pixel-switching TFT arranged on a transparent substrate, characterized in that a poly-Si TFT having an electric field-effect mobility of 1-25 cm2/V-s is used as the pixel-switching TFT, a poly-Si TFT having an electric field-effect mobility of 100 cm2/V-s or more is used as the driving element, and formation of these poly-Si TFTs and the transparent pixel electrode is conducted on the transparent substrate.
The significance of this constitution is as follows. An element having an electric field-effect mobility of 1-25 cm2/V-s makes it possible to switch the pixel at a sufficient speed. The electric field-effect mobility within this range can be obtained by a production process wherein a polysilicon layer is made at the stage of depositing silicon particles on a substrate. Thus, even if a display portion is made large, switching can be realized without dispersion.
On the contrary, the electric field-effect mobility of 100 cm2/V-s and more can be realized in a Si type TFT formed on a substrate. The electric field-effect mobility of 100 cm2/V-s or more makes necessary and sufficient high-speed control possible. Thus, the above-mentioned constitution makes it possible to provide inexpensively an array substrate for a liquid crystal display device wherein moving images can be displayed with high minuteness.
A twenty second embodiment concerned in the fifth invention group is characterized in that in the twenty first embodiment, a poly-Si TFT having an electric field-effect mobility of 1-25 cm2/V-s is used as the pixel-switching TFT, a MOS transistor having an electric field-effect mobility of 100 cm2/V-s or more is used as the driving element, and the MOS transistor is afterward mounted on the transparent substrate.
A poly-Si TFT having an electric field-effect mobility of 1-25 cm2/V-s is sufficient to turn on and turn off transmission of light. If a MOS transistor having an electric field-effect mobility of 100 cm2/V-s or more is afterwards mounted as the driving element on this poly-Si TFT, it is possible to make a TFT array substrate for a liquid crystal display device, the substrate in which the performance of the MOS transistor can be sufficiently utilized and high-frequency driving is possible.
(6) Sixth Invention Group
A twenty third concerned in the sixth invention group is a TFT array substrate for an in-plane type liquid crystal display element, comprising at least a first tandem type pixel electrode, a pixel-switching TFT for switching the first tandem pixel electrode, a driving element for driving the pixel-switching TFT, and a second tandem type pixel electrode opposite to the first tandem type pixel electrode, arranged on a substrate, characterized in that a poly-Si TFT having an electric field-effect mobility of 1-25 cm2/V-s is used as the pixel-switching TFT, a poly-Si TFT having an electric field-effect mobility of 100 cm2/V-s or more is used as the driving element, and formation of these poly-Si TFTs and the first and second tandem type pixel electrodes is conducted on the substrate.
According to this constitution, it is possible to make a TFT array for a liquid crystal display device which can be driven with high frequency. Moreover, in this substrate, angle dependence about display is little.
A twenty fourth embodiment concerned in the sixth invention group is characterized in that in the twenty third embodiment, a poly-Si TFT having an electric field-effect mobility of 1-25 cm2/V-s is used as the pixel-switching TFT, a MOS transistor having an electric field-effect mobility of 100 cm2/V-s or more is used as the driving element, and the MOS transistor is afterward mounted on the transparent substrate.
A combination of the poly-Si TFT having an electric field-effect mobility of 1-25 cm2/V-s and the afterward-mounted MOS transistor having an electric field-effect mobility of 100 cm2/V-s or more makes it possible to provide inexpensively an IPS mode TFT array for a liquid crystal display device which can be driven with high frequency and has a wide field angle.
(7) Seventh Invention Group
A twenty fifth of the present invention is characterized in that in the embodiment of the 21st, 22nd, 23rd or 24th, the pixel-switching poly-Si TFT is an n channel type and the electric field-effect mobility thereof is 5-25 cm2V-s.
The n channel type TFT has a high electric field-effect mobility. Furthermore, if a poly-Si TFT whose electric field-effect mobility is set to 5-25 cm2/V-s is used as the pixel-switching element, it is possible to make a TFT array for a liquid crystal display device having sufficient high-speed responsibility.
Of course, in the present invention, another element or other elements may be further added to each of the above-mentioned embodiments. For example, an opposite electrode (a common electrode) formed on a second substrate is made of a reflection film composed mainly of metal Al and further a color filter is formed on the surface of the opposite electrode, so that a reflection type color liquid crystal display device can be made. On the contrary, the color filter is first made on the second substrate and the opposite electrode of a transparent conductive film is formed thereon, so that a transmission type color liquid crystal display device can be made.